Conventional jet pumps include a body having three distinct regions. These regions are a converging inlet section, a mixer section of substantially uniform cross-sectional area throughout its length, and a diffuser section which diverges or increases in cross-sectional area in the flow direction. If desired, a short tailpipe having a uniform cross-sectional area equal to the cross-sectional area of the diffuser exit may be included on the end of the diffuser.
A jet pump is typically powered by a jet of fluid. A nozzle is positioned in the inlet section to convert a high-pressure stream of driving fluid into a high-velocity, low-pressure jet of driving fluid. This high velocity, low pressure jet of driving fluid flows axially through the inlet section of the jet pump and into the mixing section of the jet pump.
In virtually all jet pump applications, fluid termed as "drive fluid" is pumped to the region of the jet pump nozzle. This pumping occurs via piping of a size generally optimized to balance the captial costs of the piping against the operating costs of the pumping energy.
The flow passage of the driving fluid stream begins with the generally-always-larger cross-sectional area drive fluid supply piping, sized to mitigate fluid flow loss. At the nozzle this flow passage then gradually reduced, allowing drive flow that is initially at high pressure to accelerate smoothly until it attains the static pressure corresponding to the nozzle exit.
The drive nozzle may be comprised of a single jet or may be represented as a plurality of jets. When a single jet is used, the nozzle is positioned to discharge the jet in a downstream direction along the longitudinal axis of the jet pump body. When the drive flow is subdivided into multiple jets, these jets are usually positioned equally spaced to some radius between the jet pump body longidutinal axis and the inside diameter of the mixing section and are oriented to discharge coaxially.
The high-velocity jet or jets entrains fluid surrounding the nozzle in the inlet section as well as in the entrance region of the mixer section by conventional driving stream to driven stream momentum transfer. This momentum transfer continuously induces the surrounding or "driven" fluid to flow into and through the inlet section.
The velocity of the entrained driven fluid increases due to the decreasing cross-sectional flow area as the driven fluid moves through the converging inlet. Thus, the pressure of the combined driving and driven fluids are reduced to a low value.
The converging inlet section surrounding the nozzle directs the driven fluid into the mixing section. Within the mixing section, the high-velocity jet of driving fluid gradually widens as an entrainment-mixing process takes place with the driven fluid. During mixing, momentum is transferred from the high-velocity driving stream to the driven fluid, so pressure of the combined stream increases.
The mixing process ends in the mixer. This end occurs, in theory, after the velocity taken across an area perpendicular to the longitudinal axis of the mixer becomes nearly constant (except in the boundary layer close to the walls). When this velocity profile occurs, it is said that a nearly "flat" velocity profile has been attained. Generally, it is assumed that this flat profile occurs shortly after the jet expands to touch the walls of the mixing section.
From the mixing section, the mixed driving and driven fluids flow into a diffuser of increasing cross-sectional area in the flow direction. This diffuser has two functions.
First, it further increase inlet section to diffuser exit pump discharge pressure.
Second, the velocity of the mixed fluids exhausting from the jet pump is reduced.
Thus, a jet pump operates on the principle of the conversion of momentum to pressure. The driving fluid issuing from the nozzle has low pressure, but high velocity and momentum. By a process of momentum exchange, driven fluid from the inlet or suction section is entrained and the combined flow enters the mixing section. In the mixing section, the velocity profile, i.e., a curve showing fluid velocity as a function of distance from the longitudinal axis of the mixing section, is changed by mixing. Momentum decreases and the velocity profile becomes nearly flat, i.e., perpendicular to the longitudinal axis of the mixing chamber.
The decrease in momentum results in an increase in fluid pressure. The flat velocity profile gives minimum momentum with a resulting highest pressure increase in the mixing section. In the outwardly diverging diffuser, the relatively high velocity of the combined stream is smoothly reduced and converted to a still higher pressure.
When the term "jet pump" is used, convention implies that both suction fluid and drive fluid are in the same fluid states. The fluid states can be liquid state, or the gaseous state. When the application involves the gaseous state, convention in the continued use of the term "jet pump" implies that compressible effects are not significant in the design. Otherwise, such terms as "ejectors", "injectors", "educators", "pressure amplifiers" and the like are used to more clearly describe the application and the device characteristics.
Jet pumps are useful in many systems. Often, such system applications involve pumping large quantities of fluid at high rates. Thus, small improvements in pump performance can have major effect on system performance and economy.
One application for which liquid jet pumps are especially suited is the recirculation of coolant in a nuclear reactor of the boiling water reactor (BWR) type. In a typical large boiling water reactor about 270,000 gallons/minute of coolant is recirculated by jet pumps. Thus, it is apparent that small increases in jet pump efficiency will produce important improvements in system performance and economy.
It is desirable in certain BWRs to accomplish the nuclear reactor coolant recirculation process by forced-circulation, as opposed to natural circulation, to gain an overall more compact reactor pressure vessel with concomitant savings in nuclear steam supply system costs and containment costs. One such forced-circulation system is employed in the General Electric Company BWR/3 through BWR/6 product line of forced-circulation reactors. This system uses jet pumps mounted inside the reactor vessel.
The motive flow driving the jet pump is supplied by external mechanical (centrifugal) pumps. These external recirculation pumps take suction from the downward flow in the annulus between the core shroud and the reactor vessel wall.
This downward flow consists of feedwater mixed together with separated liquid that has been separated out from the two-phase mixture produced by the nuclear reactor core. The separated liquid is produced at the steam separator and steam dryer drains and is recirculated back to the entrance to the core. The feedwater represents coolant inventory returning to the reactor. This returning coolant inventory balances the reactor-produced steam which is supplied to the power station turbine.
In order to drive the motive flow, approximately one third of the downcomer recirculation flow is taken from the vessel through two recirculation nozzles. Thereafter, it is pumped to higher pressure, distributed in a manifold to which a number of riser pipes are connected, and returned to the vessel via inlet nozzles. Inside the reactor, piping connects from each of these inlet nozzles to one or more jet pumps.
In the jet pump this now-high-pressure flow is discharged in the jet pump nozzle, inducing the remainder of the downcomer flow. In the jet pump, the flows mix (producing exchange, and unification of momentum), diffuse (an action which converts momentum into higher pressure), and discharge into the core lower plenum. Forced circulation of the entire reactor coolant results.
One of the disadvantages of the above jet pump recirculation system is that jet pumps have characteristically poorer mechanical efficiency than do centrifugal pumps. Consequently, the electrical power (assuming motor-driven centrifugal pumps) required to drive the entire recirculation flow is greater than that for non-jet-pump recirculation systems. Those familiar with boiling water reactor design will appreciate that a non-jet-pump system often entails many other, much more costly disadvantages. Hence, the non-jet-pump system is not necessarily the indisputably preferred modern BWR recirculation system.
Certain improved BWR recirculation systems seek to eliminate the external recirculation loops associated with existing jet-pump-type BWRs. This saves capital equipment costs, enables compacting the reactor containment, and reduces the personnel radiation exposure that occurs during maintenance servicing on the drive pumps and during inservice inspections of the coolant piping weld integrity.
Among the several practical means of eliminating these external loops, one such conceptual means long under design study is to use feedwater-driven jet pumps (FWDJPs). In the FWDJP recirculation system design concept, a substantial portion--such as 80%--of the feedwater is raised to extra-high pressures--such as 2700 psig--by mechanical pumps in the feedwater train. This high-pressure feedwater is piped to the nozzles of jet pumps mounted as before in the reactor downcomer annulus. The high-pressure feedwater is accelerated in the convergent-flow-area FWDJP nozzle to high velocities and discharged at the jet pump nozzle. This induces the balance of the recirculation flow--which now consists of the mixture of liquid returning from the steam separators plus the residual (20%) portion of the feedwater--to be pumped through the FWDJP and discharged at requisite higher pressure into the core lower (entrance) plenum.
One of the disadvantages remaining with the FWDJP recirculation system described above, is that the resulting FWDJP must operate with a high proportion of induced flow per unit of drive flow. (The ratio of induced flow/drive flow is termed the "M-ratio"). A performance disadvantage with jet pumps is that when M-ratios exceed 1.5, approximately, the jet pump efficiency becomes increasingly poorer. The application described in the paragraph above produces an M-ratio of about 8.6. The FWDJP efficiency is substantially diminised below the best-possible-efficiency at which jet pumps--given lower M-ratios--are capable of operating.
Yet another disadvantage of the FWDJP recirculation system described above is that an extra mechanical pump(s) is required (if total feedwater pumping power is to be minimized) in the feedwater train(s) to boost the FWDJP drive flow beyond the 1250 psig pressure (at conventional BWR feedwater pump discharge) to the 2700 psig needed to accomplish FWDJP recirculation.
Yet another disadvantage is that piping design pressures (and thus pipe wall thicknesses and thus piping costs) are raised in the feedwater delivery piping running between feedwater pump discharge into the reactor.